The mechanisms of action of gonadotropin releasing hormone (GnRH) are being studied using both pituitary plasma membrane preparations and isolated cultured rat anterior pituitary cells. Studies using the plasma membranes are aimed at determining whether alterations in receptor number, or receptor affinity occur in different in vivo physiological situations. Changes in GnRH receptors will be correlated with changes in LH and FSH secretion in response to injections of GnRH. The GnRH induced release of LH and FSH from isolated cultured pituitary cells is used to study the effects of steroid hormones and other pituitary hormones on gonadotropin secretion. Thus, factors acting directly at the pituitary cell level, which may modulate gonadotropin release will be delineated. Similar studies using dissociated cells, in a perfusion system, will be aimed at determining the dynamics of gonadotropin release.